Once you're on the ship, enter through the door on your left. Go down the stairs on your right. On the next floor, go right, into the small corridor and enter the door on the left wall. Sprint across the sand and go down the stairs at the end of the path. Make your way through this corridor until Fi prompts you about the sealed door. Continue forward into the next room. The combination for the lock is down-up-down-right. Enter the room and get the small key. Backtrack all the way to the top and enter the locked door next to the bird statue.
Location: Lanayru Desert
Attacks: Electric Sword, Hook Swipe
Effective Weapons: Sword
Strategy: Scervo is a robotic pirate that guards the coastal area of Lanayru Desert. It fights with an electrically-charged sword, kind of like the Technoblins. One has to wonder whether the Technoblins stole their technology from the robotic guardians of this region...
Functions: Fire powerful long-range projectiles
Upgrades: Iron Bow
How to obtain: Dungeon item in the Sandship.
Description: Link's always had a bow and arrows as part of his inventory, but now they're stronger than ever. Pick off enemies from a distance, and if you're good enough you can even take them out from much closer...
Go back to the deck and hit the switch Fi showed you earlier with an arrow. Go around the mast and climb up the ladder. Use the bow to shoot the far-away switches, allowing you to zipline across. Do this until you reach the top, thrust your sword into the lock the Bokoblin locked earlier, and unlock it.Jump back down and enter the same door as before. Make your way down to the quicksand, and enter the door on the left, the one with the sunrays. Shoot an arrow through the grate on the ceiling to hit the timeshift stone. Pick up the map from the chest, pull the lever and exit. Sprint across the quicksand and shoot an arrow onto the switch through the small ventilation opening on the right wall. Enter the room to the left of the opening. There's a block to your right blocking another small opening, push it away. Stand on the switch and shoot an arrow through the opening. Head back across the quicksand and re-activate the timeshift stone. Enter the first door on the right once you're in the corridor, and thrust your sword into the machine. Activate it, and exit. Now enter the second door on the left wall. Push the block onto the switch as per usual. Go all the way back now, outside of the boat, and go to the rescue boat on the opposite end. Hit the target on the crane that holds the rescue boat to make it descend. Now hit the timeshift stone to deactivate it and drop down onto the boat. Enter the room from here, and go to the section with the generator. Hit the timeshift stone from the opening in the ceiling to reactivate it and turn on the generator. Enter the room with the two windows again and drop down the left window, down the ladder. Enter the engine room and jump down.
Carefully make your way past the three pistons. Grab hold of the pole with your whip, then turn to the side and swing yourself onto the small ledge. Climb up and collect the goods. Go back down and drop in front of the next set of pistons. As soon as they raise, just sprint through. Wait for the closest piston now to drop down, then swing yourself on top of it. In order to reach one in front now, you have to wait until it drops down, but the one you're on is up. That way you'll be able to jump on it. Pull the lever to open the gate, then go ahead and crawl through the small opening. Pull the lever and talk to the robot. Now that you have the key, crawl back into the engine room and simply go forward and follow the corridor to reach the exit door and go back up to the ship. Head up to the deck and deactivate the timeshift stone. Enter the locked door at the opposite side of the ship, where the rescue boat is, and go down to the room with the grate in the ceiling. Hit the timeshift stone from there and kill the enemies. Stand on the switch and shoot an arrow through the small opening in the wall to open the door. Kill the Beamos here (Use the bow) and take the Squid Carving from the chest.
Go back into the main section of the ship, down to the control room door. Rotate the carving into place and enter the boss room.
Tentalus is most vulnerable to shots from your Bow immediately after launching an attack. You can move around while aiming your Bow, so use the mobility to your advantage. Dodge a tentacle swipe and fire! Hitting the tentacles will replenish your life, so it's usually worth it to attempt to attack them rather than trying too hard to avoid damage.
After a while the fight will switch to another portion of the ship. This time Tentalus will attack you with his head tentacles. Your best bet is to just slice at them with your sword as they come, making sure none of them get you.